As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has been practically employed. The method employs a radiation image storage panel comprising a support and a stimulable phosphor layer (stimulable phosphor sheet) provided thereon, and comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The panel thus treated is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In the above method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resource and economic efficiency because the radiation image storage panel can be repeatedly used while the radiographic film is consumed for each radiographic process in the conventional radiography.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. If the phosphor layer is self-supporting, the support may be omitted. The phosphor layer usually comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor with no binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, the layer comprising agglomerated phosphor soaked with a polymer is also known.
The stimulable phosphor emits stimulated emission when excited with stimulating rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the from of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the storage panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
In general, a transparent film of polymer material is placed on the free surface (surface not facing the support) of the phosphor layer to keep the layer from chemical deterioration or physical shock. This surface protective film can be formed by various method, for example, by applying a solution of resin (e.g., cellulose derivatives, polymethyl methacrylate), by fixing a transparent resin film (e.g., a glass plate, a film of organic polymer such as polyethylene terephthalate) with adhesive, or by depositing inorganic materials on the phosphor layer.
In order to improve the quality (e.g., sharpness, graininess) of the resultant visible image, a radiation image storage panel having a protective film of a particular haze is proposed in Japanese Patent Provisional Publication No. 62 (1987)-247298. Further, the inventors proposed a storage panel having a new protective film (U.S. Ser. No. 09/050,953, now allowed). The proposed film has a multi-layered structure comprising a plastic film and a fluorocarbon resin layer containing light-scattering fine particles.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image, and it is desired for the radiation image storage panel employed in the method to have a high sensitivity and give an image of high quality (such as high sharpness and high graininess).
The radiation image storage panel is repeatedly used in the cyclic procedure comprising the steps of: exposing to a radiation (for recording of a radiation image), irradiating with stimulating rays (for reading of the recorded image), and exposing to an erasing light (for erasing the remaining image). In this procedure, the storage panel is transferred from one step to another by means of conveying means such as belt and rollers in the radiation image recording and reproducing apparatus, and after a cycle of the steps is conducted, the storage panel is piled up on other storage panels and stored for next cycle. Since the surface of the storage panel is directly brought into contact with the conveying means (e.g., belt and rollers), stains and abrasions are liable to be produced. The stains and abrasions thus produced on the protective film disturb passage of the stimulating ray and/or the stimulated emission, and consequently depress the resultant image quality. For this reason, the surface of the panel has to have enough durability to resist the stains and abrasions.
Hitherto, the sharpness of resultant image has been thought to be improved by thinning the protective film. The thin protective film, however, often cannot satisfactorily protect the panel from the stains and abrasions, and hence the storage panel with the thin protective film generally has unsatisfactory durability. In order to solve this problem, various protective films were proposed. For example, a material having both high transparency and enough strength (e.g., polyethylene terephthalate) can be employed, or some kinds of resins can be used in combination. Further, a protective film having a multi-layered structure is also known.
Those known protective films have been developed in consideration of protection of the stimulable phosphor layer from chemical and physical deterioration (e.g., scratch resistance, stain resistance and abrasion resistance), as well as sharpness of the resultant image. However, although those protective films are improved to a certain extent, their properties should be more improved.